The major aim of this research is to examine the role of guanosine 3',5' cyclic monophosphate (cGMP) in the regulation of myocardial calcium channels. Whereas cAMP is known to play a major role in cardiac function, the role of cyclic GMP remains unclear and controversial. However, recent results indicate that cyclic GMP may mediate some of the cardiac effects of physiologically (and pathophysiologically) important compounds (e.g., acetylcholine). Thus, the first hypothesis to be tested is that cGMP is an important modulator of the cardiac calcium (I-Ca). The physiological relevance of cGMP inhibition of I-Ca will be assessed by (1) examining the effect of raising and lowering cGMP levels on I-Ca, and (2) by alternately blocking and enhancing the cGMP-stimulating action of several physiologically important endogenous compounds in order to determine whether cGMP plays a role in mediating some of the effects of these compounds. The mechanism of cGMP inhibition of I-Ca will also be studied. Thus, the effects of (1) cGMP analogs, (2) intracellular application of cGMP, and (3) intracellular application of activated cGMP- dependent protein kinase on I-Ca will be assessed. Additionally, the effects of many of the interventions of cAMP and cGMP levels and cAMP- and cGMP-dependent protein kinase activities will be examined and correlated with effects on I-Ca. Furthermore, the possible interaction of cGMP and cAMP in the modulation of I-Ca will be examined by determining whether the effect of cGMP requires, or is modified by, the presence of elevated [cAMP]i, as well as the reverse (i.e., whether [cGMP]i modifies the effects of cAMP on I-Ca). These experiments, which will be undertaken using the patch clamp technique on ventricular myocytes from guinea pigs and embryonic chickens, should answer not only the questions of how and under what conditions cGMP regulates I-Ca in cardiac cells, but whether such regulation has any physiological significance. Such information is important in enhancing our knowledge about subcellular regulation of cardiac function, as well as contributing to our understanding of the mechanisms of action underlying the cardiac effects of certain endogenous compounds.